In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The detectors are generally rectangular. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. Typically, the configuration of a slice may be varied. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
In known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray absorbing material with an aperture therein for restricting the x-ray beam. Known apertures are typically linear, or rectangular, and the aperture width controls the slice thickness as measured along the z-axis. For example, by passing an x-ray beam through a collimator with a 1 mm aperture, the beam output from the collimator will have a 1 mm thickness.
Known CT systems typically provide adequate image resolution. However, such resolution is limited, for example, by the collimator size, the slice thickness and the filter kernel. With respect to 3D and multi-planar reformat (MRP) images, it would be desirable to improve image resolution for all collimator sizes.
Image resolution is related to slice thickness. Particularly, by reducing the slice thickness, the image resolution is improved. In some applications, a slice thickness as thin as 0.5 mm is desired. Known CT systems, however, typically are configured to provide the smallest slice thickness of 1 mm. Until now, it was believed that in order to reduce the slice thickness to 0.5 mm, significant hardware and software changes were necessary.
Image resolution also is related to the reconstruction filter kernel. Particularly, increasing the cut-off frequency of the filter kernel causes an improvement in image resolution in the x-y plane. However, increasing the filter kernel cut-off frequency also increases the high frequency contents contributing to an image, and causes significant aliasing artifacts. Accordingly, the reconstruction filter kernel in a known CT system typically must be limited. Until now, it was believed that in order to further increase the reconstruction filter kernel cut-off frequency, significant hardware and software changes were necessary.
It would be desirable to improve image resolution in a CT system by providing a slice thickness as thin as 0.5 mm. It also would be desirable to improve image resolution by facilitating the use of even higher reconstruction filter cut-off frequencies. It also would be desirable to provide such image resolution without degrading overall image quality, and without requiring significant hardware and software changes in known CT system.